This invention relates to apparatus for the separation of both magnetic and nonmagnetic solids from a liquid such as, typically, printing ink by means of magnetic attraction and filtration. The solid-liquid separation apparatus in accordance with the invention also lends itself to the processing of petroleums, organic solvents, paints, dyes, etc., and to water purification.
Let us consider gravure printing as an illustration of the need for the removal of solid particles from printing ink. In gravure printing the image area is etched, engraved or otherwise created below the surface of the printing plate or cylinder in the form of tiny "cells" or depressions. The printing surface is flooded with ink and then wiped or scraped clean of excess ink by a steel blade, called a doctor, while the cells hold the ink below the surface. The ink is transferred from the cells to paper pressed against the surface.
If the gravure printing ink contains solid matter, therefore, it will become caught between the printing surface and the doctor and so prevent the proper functioning of the latter. Further, the arrested solids will cause the rapid or uneven wear and damage of both the printing surface and the doctor, materially shortening their useful life. The worm or damaged printing surface and doctor also impair the quality of the reproductions and even produce defective prints that must be discarded. These results greatly increase the manual labor of removing the arrested solids from between the printing surface and the doctor, reconditioning or replacing them, and inspecting the produced prints for defective ones. The consequent increase in printing costs is also a problem.
A complete removal of solid particles from gravure printing ink is the only possible solution to the foregoing problems. A variety of solid-liquid filters or separation devices have been suggested and used in the printing industry. Typical of these are meshed or screen filters, roll-up filters, stacked-ring filters, and a combination of a screen filter and magnetic means. All these devices have one drawback or another.
Screen filters rapidly deteriorate in their filtration capacity with the clogging of the pores. They must therefore be backwashed periodically to make them clean of accumulated solids, with the filtering operation suspended. Roll-up filters are easily ruptured by the liquid pressure. Being for a single use, moreover, they are expensive and also require a break in the filtering operation for the replacement of used rolls. Stacked-ring filters are available with facilities for reconditioning without interruption of filtering. However, they are limited in the size of particles that can be filtered off, and so are unable to capture fine iron or other metal particles which do much harm in gravure printing.
Known apparatus comprising both a screen filter and magnetic means for solid-liquid separation is also subject to several objections. One is the arrangement of the magnetic means in relation to the flow path of the liquid. As all of the liquid does not pass sufficiently close to the magnetic means in the prior art apparatus, the magnetic matter contained in the liquid is not separated therefrom to a desired degree. The screen filter for combined use with the magnetic means has its own disadvantages pointed out previously.